A kinetic study about the reactions of NH(X3Σ−) with hydrocarbons part 1: Saturated hydrocarbons and acetaldehyde

The Reaction NCN + H2: Quantum Chemical Calculations, Role of 1NCN Chemistry, and 3NCN Absorption Cross Section

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. Recently, in a combined shock tube and flame modeling study, the so far neglected reaction NCN + H₂ and the related chemistry of the main product HNCN turned out to be significant for NO modeling under fuel-rich conditions. In this study, the reaction has been thoroughly revisited by detailed quantum chemical rate constant calculations both for the singlet ¹NCN and triplet ³NCN pathways. Optimized geometries and vibrational frequencies of reactants, products, and transition states were calculated on B3LYP/aug-cc-pVQZ level with single-point energy calculations carried out against the optimized structures using CASPT2/aug-cc-pVQZ. The determined rate constants for the ¹NCN + H₂ reaction as well as the newly measured high temperature absorption cross section of ³NCN made a reevaluation of the shock tube data of the previous work necessary, finally revealing quantitative agreement between experiment and theory. Moreover, the new directly measured Doppler-limited absorption cross section data, σ(³NCN, λ = 329.1302 nm) = 2.63 × 10⁹ × exp(-1.96 × 10^-3 × T/K) cm²/mol (±23%, p = 0 bar, T = 870-1700 K), are in agreement with previously reported values based on detailed spectroscopic simulations. Hence, a long-standing debate about a reliable high temperature ³NCN absorption cross section has been resolved. Whereas ³NCN + H₂ resembles a simple abstraction type reaction with the exclusive products HNCN + H, the singlet radical reaction is initiated by the insertion into the H-H bond. Up to pressures of 100 bar, the main products of the subsequent decomposition of the H₂NCN intermediate are HNCN + H as well, with minor contributions of CN + NH₂ toward higher temperatures. Although much faster than the triplet reaction, the singlet radical insertion is actually rather slow, due to the necessary reorganization of the HOMO electron density in ¹NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of ¹NCN and ³NCN chemistry. However, as the main reaction products in case of the H₂ reaction are the same and as the population of the ¹NCN in thermal equilibrium remains low, a properly weighted effective rate constant k(NCN + H₂ → HNCN + H) = 2.62 × 10⁴ × (T/K)^2.78 × exp(-97.6 kJ/mol/RT) cm³ mol^-1s^-1(±30%, 800 K < T < 3000 K, p < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for ¹NCN chemistry.

Observation of NH X(3)Σ(-) as a Primary Product of Methylamine Photodissociation: Evidence of Roaming-Mediated Intersystem Crossing?

3+1 Resonance-enhanced multiphoton ionization and photofragment excitation spectroscopy have been used to identify NH X(3)Σ(-) as a primary product of methylamine photodissociation after state-specific excitation to the S1 state. On the basis of standard thermochemical data, NH X(3)Σ(-) can be formed only in conjunction with closed-shell CH4 coproducts, indicating that dissociation must occur on the T1 surface. It is proposed that the mechanism for the formation of triplet NH and CH4 involves intramolecular abstraction between frustrated radical products and is an example of roaming-mediated intersystem crossing.

Thermal decomposition of HN(3)

The two-channel thermal decomposition of hydrogen azide, HN(3), was studied computationally. The reaction produces triplet or singlet NH and N(2). A model of the reaction was created on the basis of the theoretical study of the reaction potential-energy surface and microscopic reaction rates by Besora and Harvey (Besora, M.; Harvey, J. N. J. Chem. Phys. 2008, 129, 044303) and the experimental data on the energy-dependent rate constants reported by Foy et al. (Foy, B. R.; Casassa, M. P.; Stephenson, J. C.; King, D. S. J. Chem. Phys. 1990, 92, 2782) The properties of the model were adjusted to fit the calculated k(E) dependence to the experimental one. The experiments on thermal decomposition of HN(3) described in the literature were analyzed via kinetic modeling; the results of the analysis demonstrate that all but one of the existing studies were affected by contributions from secondary kinetics. The model of the reaction was then used in master-equation calculations of the pressure effects and the value of the critical energy transfer parameter, DeltaE(down), was adjusted based on agreement with the experimental k(T,P) data. Finally, the model was used to determine pressure- and temperature-dependent rate constants for both channels of reaction 1, which do not conform to the traditional formalism of low-pressure-limit and falloff description. Uncertainties of the model and their influence on the calculated thermal rate constant values were analyzed. Finally, parametrized expression for rate coefficients were provided for a wide range of temperatures and pressures.

Conformation and Aggregation of Proline Esters and Their Aromatic Homologs: Pyramidal vs. Planar RR´N-H in Hydrogen Bonds

The conformations of proline esters are investigated by infrared spectroscopy in supersonic slit jet expansions. Two easily convertible puckering variants of the pyrrolidine ring with intramolecular N-H···O contacts are shown to be particularly stable. The aggregation tendency of proline esters via intermolecular N-H···O hydrogen bonds is remarkably weak. IR differences between enantiopure and racemic dimers are difficult to quantify. Dehydrogenation of the pyrrolidine ring to pyrrole leads to a stable planar carboxylic ester conformation. Its aggregation tendency is pronounced due to the planar hybridization of the nitrogen atom and leads to a symmetric, β sheet-like dimer with strongly red-shifting hydrogen bonds. The spectroscopic observations underscore the differences between intermolecular interactions of N-terminal and peptide-bound amino acids in peptide chains.

Low temperature NH(X 3sigma-) radical reactions with NO, saturated, and unsaturated hydrocarbons studied in a pulsed supersonic laval nozzle flow reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.